Unequal cell division is a central mechanism in the generation of new cell types. In Caulobacter crescentus differentiation is the direct result of repeated, asymmetric cell division and this aquatic bacterium provides an ideal model for elucidating the mechanisms underlying developmental control. The patterns of differentiation and cell cycle progression in these cells are closely coordinated by two- component signal transduction systems. Possibly unique aspects of these two-component proteins is the fact that they respond to internal cell cycle cues instead of environmental conditions and some of them are essential for division and cell viability. The organization of the signal transduction pathways is not fully defined, however, and how these pathways coordinate the precise temporal and spatial patterns of developmental events during the cell cycle is far from clear. Experiments described in the proposal are designed to address the following goals: I. Identify and characterize components in the essential signal transduction pathway, DivJ => DivK => X => CtrA, which regulates cell cycle progression and differentiation; 2. Determine how the essential response regulator DivK functions in a signal transduction pathway(s) to control both cell division and motility; 3. Characterize mechanism of action and function of tyrosine kinase DivL in signal transduction; 4. Analyze the roles of histidine kinase DivJ and response regulator PleD in establishing cellular polarity; and 5. Examine the regulation of cell division genes in vivo by the essential signal transduction pathway responsible for phosphorylation and activation of CtrA.